Coriolis type apparatus for measuring mass flow of a fluid stream

ABSTRACT

A Coriolis type apparatus for measuring mass flow of a fluid stream, comprising at least one flow conduit which is oscillated at a resonant frequency transverse to the direction of flow while said fluid flows therethrough, two sensors for generating analog signals corresponding to the motion of the flow conduit, and signal processing means for determining time difference between said analog signal, said signal processing means including means for converting said analog signals into series of digital signals and means to compute from said series of digital signals said time difference by using Fourier transformation. Each of the analog signals generated by the sensors is converted into a series of digital signals by an incremental type analog-digital converter having a comparator, an up-down counter and a digital-analog converter.

TECHNICAL FIELD

The invention relates to a Coriolis type apparatus for measuring massflow of a fluid stream, comprising at least one flow conduit which isoscillated at a resonant frequency transverse to the direction of flowwhile said fluid flows therethrough.

BACKGROUND ART

Mass flow measuring apparatus utilizing the effect of Coriolis force arebased on the physical phenomenon, that Coriolis force acts on a conduitif it is vibrated perpendicular to the direction of flow through theconduit. The magnitude of the Coriolis force depends on the angularvelocity of the vibration and the mass flow. The Coriolis force is aperiodic force with a frequency equal to the frequency of vibration. Asa consequence, a phase difference (time lag) in motion is obtained if aconduit fixed at two points is vibrated at its centre between the twosupports and the motion is measured by sensing devices at two pointslocated symmetrically at both sides of the centre, which results fromthe superposition of the vibrating force and the Coriolis force and isproportional to the mass flow. Signal processing units of mass flowmeters calculate the mass flow on the basis of signals of sensingdevices located symmetrically. The design and operating characteristicsof Coriolis mass flow meters; are closely related to the conduitassembly used.

The mass flow information is always carried by the component caused bythe Coriolis force in the electric signal supplied by the sensingdevices that are located symmetrically to the centre of the conduit. Thesignal processing units measure either this specific component oranother parameter traceable to this. Such parameters are e.g. time lagbetween the two signals, the integral formed from the absolute values ofsignal differences for a finite number of full periods etc.

Signal processing units of this kind are described in British Patent No.2,171,200, U.S. Pat. No. 4,879,911, Hungarian Patent No. 200,234,International Patent Application No. WO 88/03261 and U.S. Pat. Nos.4,655,089 and 4,996,871.

The apparatus described in the British Patent No. 2,171,200 is connectedto a conduit assembly having a pair of straight conduits parallel toeach other. The effect of the interfering longitudinal (axial) stressesgenerated during the vibration of the straight conduit sections areeliminated by generating the vibration at the fundamental resonantfrequency and its third harmonic frequency simultaneously, whileutilizing the property of the pair of parallel conduits that the ratioof the fundamental frequency to its third harmonic carries informationabout the axial stresses. In a first approach, the mass flow will bedetermined on the basis of the phase difference obtained at thefundamental frequency and, then, the result thus obtained will becorrected taking the measured ratio of the fundamental frequency to itsthird harmonic into account.

The apparatus described in U.S. Pat. No. 4,879,911 reduces the measuringtask to the measurement of time difference. By analog integration of thesignals of the sensing devices a reference signal and a measuring signalproportional to the displacement will be obtained. By comparing thesesignals with zero-symmetric reference voltages, gate-times are obtained,that contain the time lag between the signals with various signs. Bysumming the gate times with proper signs, the time lag between thesignals will be obtained, which is proportional to the mass flow.

The apparatus described in Hungarian Patent No. 200,234 uses theintegral value of the signals of the sensing devices to calculate themass flow.

By adding the frequency component associated with the deformation causedby the Coriolis force to the driving signal, the effect of Coriolisforce will be significantly increased. This is utilized by the apparatusdescribed in International Patent Application No. WO 88/03261, in whichfour independent driving and sensing elements are used. The signalcaused by the Coriolis force and the driving signal, contained in thecomplex input signals, are separated by means of synchronous rectifiersand the mass flow is calculated on the basis of the ratio of these twocomponent signals.

The apparatus described in U.S. Pat. No. 4,655,089 uses an integratorwith coupled capacitors to generate signals proportional to thedisplacement. The signals thus obtained will be fed to a voltagecomparator each of variable reference, which control a phase comparator.The phase comparator sets the output voltages of digital-analogconverters through a digital counter so as to obtain zero (or constant)time delay at the input of the phase comparator while using the saidoutput voltages as references to the said voltage comparators. In theequilibrium once obtained, the difference between the control codes ofdigital-analog converters will be proportional to the mass flow.

In U.S. Pat. No. 4,996,871 a Coriolis densimeter is described whichrelies on measuring mass flow rate by determining the phase differencethat occurs between real and imaginary components of the discreteFourier transformation of signals of two velocity sensors attached tothe conduits vibrated. The analog signals of the sensors aremultiplexed, filtered with an anti-aliasing low pass filter, thensubject to a sample and hold function and digitalized in order toperforme the discrete Fourier transformation. Alternatively, a separatelow pass filter could be situated in front of the multiplexer for eachof the two incoming velocity signals in lieu of the anti-aliasingfilter. Unfortunately, the use of such filters may result inmeasurements error as their characteristics vary due to temperaturevariations and thereby influencing the phases of the signal to bemeasured. Moreover, in the apparatus described the samples of the twosignals are interleaved, i.e. they cannot be sampled at the same time.This phase shift has to be compensated which is rather complicated andrepresents a further source of measurement error.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide an improved Coriolistype mass flow meter using Fourier transformation for determining themass flow rate.

Hence, the invention is a Coriolis type apparatus for measuring massflow of a fluid stream, comprising at least one flow conduit which isoscillated at a resonant frequency transverse to the direction of flowwhile said fluid flows therethrough, two sensors for generating analogsignals corresponding to the motion of the flow conduit, and signalprocessing means for determining time difference between said analogsignals, said signal processing means including means for convertingsaid analog signals into series of digital signals and means to computefrom said series of digital signals said time difference by usingFourier transformation. According to the invention each of the analogsignals generated by the sensors is converted into a series of digitalsignals by an incremental type analog-digital converter having acomparator, an up-down counter and a digital-analog converter, theup-down counter of both analog-digital converters are driven by a commonclock signal source, and said series of digital signals are forwarded tothe computing means from outputs of the up-down counters throughcontrolled latches.

The incremental type analog-digital converters used in the apparatusaccording to the invention obviate the use of analog filters asdescribed in the cited prior art. Further, the frequency characteristicsof an incremental type analog-digital converter can easily be set byadjusting the frequency of the clock signal.

It is a further advantage of the apparatus according to the inventionthat both analog signals generated by the sensors can be sampled in thesame time.

An embodiment of the apparatus according to the invention ischaracterized in that for generating the control signal for the latchesit comprises a reference oscillator including a comparator having aninput connected to one of said analog signals, a phase detector, a lowpass filter and a voltage controlled oscillator connected in series, anda binary divider driven by the voltage controlled oscillator and havingan output connected to an input of the phase detector. With thisarrangement the frequency of the signal controlling the latches caneasily be set to an integral multiple, e.g. 16-times, of said resonantfrequency.

According to a further aspect of the invention each of the incrementaltype analog-digital converters is designed so that LSB/T≧2·√2·π·U·f,where LSB is the least significant bit of the up-down counter, T is thetime period of the clock signal, U is the r.m.s. value of the greaterone of said analog signals, and f is the frequency of said analogsignal.

The apparatus according to invention comprises vibrating means tooscillate the flow conduits and it is advantageous if the vibratingmeans are driven by a signal which is in phase of the sum of said analogsignals generated by the sensors.

Further advantages of the invention will become apparent by particularlypointing out preferred embodiments of the invention. For the purpose ofillustrating the invention, there is shown in the drawings a form whichis presently preferred; it being understood, however, that the inventionis not limited to the embodiments shown.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an overall diagram of the Coriolis type apparatus accordingto the invention comprising a perspective view of the conduit assemblyand a simplified block diagram of the signal unit.

FIG. 2 is a block diagram of an embodiment of the signal unit as shownin FIG. 1.

FIG. 3 is a diagram showing the sampling of two analog signals generatedby two sensors according to the invention.

MODES FOR CARRYING OUT THE INVENTION

In the drawings like reference numbers indicate like elements.

FIG. 1 shows an embodiment of a mass flow metering apparatus accordingto the invention which includes a conduit assembly 10 and a signal unit30. The conduit assembly 10 as shown includes an inlet part 11, anoutlet part 12, a central mounting block 13 and two generally B-shapedflow conduits 14 and 15. The inlet part 11 and the outlet part 12includes flanges 16 and 17, respectively, which are formed inanticipation of mounting the conduit assembly 10 within a defined fluidstream of pipeline (not shown). The inlet part 11 and the outlet part 12generally define an inlet and outlet to the flow conduits 14, 15 fromthe fluid stream and preferably positioned coaxially with respect to oneanother as well as coaxial with the longitudinal axis 18 of the fluidstream to be measured.

The mounting block 13 generally forms an inlet channel (not shown),which communicates with the inlet part 11, and an outlet channel (notshown), which communicates with the outlet part 12. The inlet channelgenerally forms a flow splitter to divide the fluid flow passing throughthe mounting block 13 so as to feed substantially equivalent portions ofthe flow from the inlet part 11 into each of the two flow conduits 14,15. The flow passes through the two flow conduits 14, 15 in asubstantially simultaneous and parallel relationship. A single exhaustflow is formed at a flow converger in the outlet channel and is directedinto the outlet part 12.

The flow conduit 14 has an inlet end 19 and an outlet end 21 which arefixedly attached to the mounting block 13. Similarly, inlet end 20 andoutlet end 22 of the flow conduit 15 are fixedly attached to themounting block 13. The two flow conduits 14 and 15 are fixed to eachother by two brackets 23 and 24 adjacent the respective mounting ends19, 20 and 21, 22.

A vibrator 25 is mounted to a position between the two substantiallystraight portions of the flow conduits 14 and 15 by means of an arm 26in order to oscillate the flow conduits 14 and 15 in an opposite phasewith respect to each other. Sensors 27 and 28 are positioned on arcuateportions of the flow conduits 14 and 15 so as to measure the motion ofthe flow conduits 14 and 15 with respect to one another. The vibrator 25and the sensors 27, 28 may take any desired form as known in the art.

The signal unit 30 determines mass flow through the flow conduits 14, 15as a result of analog signals received from the sensors 27 and 28 vialeads 32 and 33, respectively. Further, the signal unit 30 generates adriving signal on lead 31 for the vibrator 25. The signal unit 30delivers a mass flow rate signal on its output 34.

Clearly, those skilled in the art recognize that, although the disclosedembodiment of the conduit assembly utilizes generally B-shaped flowconduits, one flow conduit or more flow conduits of almost any size andshape may be used as long as the conduit or conduits can be oscillatedabout an axis transversal to the fluid stream within the conduit.

FIG. 2 shows a block diagram of the signal unit 30. The analog signalsUA and UB from the sensors 28 and 27, respectively, will be fed to inputstages 40 and 41, respectively. The input stages 40, 41 amplify theanalog signals to a level of about 5 V r.m.s. The amplified analogsignal at the output of the input stage 40 is fed to the input of the anincremental type analog-digital converter 50 the output of which isconnected to a controlled latch 52. The analog-digital converter 50includes an input comparator 42, an up-down counter 44 and adigital-analog converter 46 supplied the same reference voltage Uref.Similarly, another incremental type analog-digital converter 51 isconnected to the output of the input stage 41, and the output of thisconverter 51 is connected to another controled latch 53. Theanalog-digital converter 51 includes an input comparator 43, an up-downcounter 45 and a digital-analog converter 47 supplied by a referencevoltage Uref. The operation of the analog-digital converters 50 and 51will be explained by reference to the analog-digital converter 50, only.

The signal from the input stage 40 will be fed to the "+" input of thecomparator 42 which, in turn, controls the Up/Down input of the up-downcounter 44. The output code of this counter 44 sets the output voltageof the digital-analog converter 46. This output voltage will be fed tothe "-" input of the comparator 42. The comparator 42 compares the twovoltages at its inputs and controls the Up/Down input of the counter 44so as to make the output voltage of the digital-analog converter 46equal to the output signal from the input stage 40 at any time. Theclock signal necessary for the operation of the counter 44 will besupplied by a clock generator 48 through a control logic unit 49. Thefrequency of the clock generator 48 can be determined from the zerotransition slope of the output signal from the input stages 40, 41 andthe minimum voltage change (LSB) of the analog-digital converters 50,51. Each of the incremental type analog-digital converters 50 and 51 isdesigned so that

    LSB/T≧2·√2·π·U·f,

where LSB is the least significant bit of the up-down counter, T is thetime period of the clock signal, U is the r.m.s. value of the greaterone of said analog signals, and f is the frequency of the analog signal,i.e. the resonant frequency of the flow conduits 14, 15. Preferably, thevalue of LSB/T is only slightly greater than 2·√2·π·U·f.

The output code of the analog-digital converter 50 is stored in thelatch 52. The sampling is performed by writing the output code to thelatch 52 at appropriate times. In this circuit an analog sample and holdcircuit is not necessary. The measuring times and the phase states arestored in a 6-bit latch 58. The control logic unit 49 controls the loadinput of the latches 52, 53 taking the phase of the reference oscillator60 into consideration. A computing unit 55, provided with CRAM 56 andPROM 57, is connected to the latches 52, 53 via bus 54. The computingunit 55 performs the reading of the content of the latches 52, 53 aswell as the calculations according to Fourier transformation describedbelow.

For sampling the amplified signals there is a reference oscillator 60including, in series, a zero level comparator 61 having an inputconnected to one of the amplified signals, a phase detector 62, a lowpass filter 65 having a resistor 63 and a capacitor 64, and a voltagecontrolled oscillator 66. The output of the voltage controlledoscillator 66 is connected to the input of a binary divider 59 havingsix outputs. The output of the most significant bit is connected toanother input of the phase detector 62.

The output frequency of the reference oscillator 60 shall be set to avalue that is an integer multiple of the frequency of the analog signalsUA and UB. It is recommended to set the frequency of the referenceoscillator 60 to a value equal to four times the sampling frequency, asin this case the control signals that will be needed for signalprocessing will also be available. The sampling frequency may be equalto 16-times the frequency of the analog signals UA and UB. This controlis performed by the zero level comparator 61, the phase detector 62 andthe low pass filter 65. The zero level comparator 61 is connected to theoutput of the input stage 41. The most important element of the controlcircuit is the phase detector 62, the "-" input of which receives theoutput signal from the zero level comparator 61, while the "+" inputreceives the signal from the voltage controlled oscillator 66 subdividedaccording to the powers of 2 by the binary divider 59. Based on thesignals fed to its input, the phase detector 62 determines the phases ofthe two signals and issues a charge of proper sign to the low passfilter 65 for the time proportional to the phase difference. The lowpass filter 65 will convert the charge into voltage and stores it untilthe next comparison. The output of the phase detector 62 is in ahigh-impedance state until a new comparison takes place. The voltagefrom the low pass filter 65 controls the voltage-controlled oscillator66 which, in turn, sets the sampling times by means of the binarydivider 59.

In FIG. 2 a preferred embodiment for the driving circuite of thevibrator 25 is also shown by which the distortion due to the Coriolisforces are eliminated. The analog signal UA is fed through the inputstage 40 to one of the inputs of a summing circuit 35, while the analogsignal UB through the input stage 41 to the other input of the summingcircuit 35. The output signal of the summing device 35 is connected toone input of an amplitude controller 36, while its other input receivesa reference signal UrefC of appropriate value. The amplitude controller36 amplifies (or deamplifies) the signal received from the summingcircuit 35 to an extent that the desired input amplitude will be set bycontrolling the vibrator 25 through a power stage 37. Finally, theamplitude controller 36 sets the output signal of the summing device 35to the value "UrefC", which, at the same time, results in that theoutput voltage amplitude of the input stages 40 and 41 will also bestabilized. The value "UrefC" is chosen so that the measuring ranges ofthe digital-analog converters 46 and 47 may be used as far as possible.

The sensors 27 and 28 supply analog signals that contain the signalsgenerated by the Coriolis-force in opposite phases; thus, the lattersignal components are not present in the sum of the two signals but toan insignificant extent. By using the summed signal to control thevibrator 25, the flow conduits 14 and 15 will always be vibrated attheir resonance frequencies. In addition to a reduced driving powerrequirement, this method also involves the advantage that the accuracyof density measurement will be improved due to the fact that the valueof resonance frequency is used in density calculation.

In the following the method of processing the signals of the apparatusaccording to the invention by using Fourier transformation will beexplained. Reference is made to FIG. 3 where voltage signal UA denotedby reference number 38 and voltage signal UB denoted by reference number39 are shown.

By vibrating the flow conduits 14, 15 of a mass flow meter, periodicvoltages of the same frequency will be induced in the sensors 27, 28.The measuring task consists in an accurate measurement of the time lagbetween the two signals UA and UB. The periodic voltage signals UA andUB can be expanded into Fourier series. The Fourier series can be ofarbitrary length, however, due to the fact that the frequency of theCoriolis force is equal to that of the driving force, it is sufficientto use the series relating to the fundamental frequency:

    UA(ω·t)=aO1+a1·cos (ω·t)+b1·sin (ω·t) (1)

    UB(ω·t)=aO2+a2·cos (ω·t)+b2·sin (ω·t) (2)

where

UA, UB Fourier series of fundamental frequency of the two voltagesignals supplied by the two sensors;

aO1, aO2 DC components;

a1, a2 Fourier coefficients of the cosinus-term;

b1, b2 Fourier coefficients of the sinus-term.

The DC components of the signals of the sensors 27, 28 carry noinformation, therefore, it is not necessary to be calculated. Equations(1) and (2) can be rewritten in the form as follows:

    UA(ω·t)=CA·sin(ω·t+φA) (3)

    UB(ω·t)=CB·sin(ω·t+φB) (4)

where ##EQU1## CA, CB peak-value of signals UA and UB ##EQU2##

From the equation (6): ##EQU3##

where

φA, φB phases of signals UA and UB.

In the case of mass flow signal processing, it is the time difference δTbetween the two signals UA and UB from the two sensors 27, 28 that carrythe necessary information. This time difference δT can be calculated byusing the two phase angles and the period: ##EQU4##

where

δφ phase difference between the signals UA and UB from the two sensors27, 28;

δt time difference between the signals UA and UB;

T time period of the signals UA and UB.

For the purpose of subsequent signal processing, it is recommended tofilter the time difference values δt by means of digital filter, whichenable the deviation of measuring results to be reduced.

When using the method described, the Fourier coefficients a1, b1, a2, b2used in the equation (6) shall be determined. These coefficients can bedetermined by using the approximative harmonic analysis. For thispurpose, the periodic function shall be divided into 2·n equal parts.Due to the binary representation used in digital computers, it isrecommended to perform the decomposition by powers of 2. In selectingthe number of parts, it shall be taken into account that a small numberof parts involves a rough resolution in the results. On the other hand,large number of parts will be limited by the time necessary for thecalculation of measuring results. Taking these aspects intoconsideration, the decomposition into 16 parts seems to be sufficient,which is shown in FIG. 3. The Fourier coefficients shall be determinedby using the method of least squares. By omitting detailed explanation,the equations to calculate the Fourier coefficients are as follows:##EQU5##

where

UAi, UBi voltage values of signals UA ans UB measured at the time i.

For the calculations, the voltage of both signals shall be measured atequal intervals. For the generation of equal intervals, a referenceoscillator is needed, which operates at a frequency equal to 16-times ofthe frequency of the input signal. The time period of the referenceoscillator determines the sampling time for both input signals. Thecoefficients of the Fourier series shall be calculated by using theequations (9) to (12).

The present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof and,accordingly, reference should be made to the appended claims, ratherthan to the foregoing specification, as indicating the scope of theinvention.

We claim:
 1. A Coriolis type apparatus for measuring mass flow of afluid stream, comprising:at least one flow conduit which is oscillatedat a resonant frequency transverse to the direction of flow while saidfluid flows therethrough, two sensors for generating two analog signalscorresponding to the motion of said flow conduit, two incremental typeanalog-digital converters for converting said analog signals into seriesof digital signals, each of said incremental type analog-digitalconverters comprising a comparator, an up-down counter and adigital-analog converter, a common clock signal source for driving saidup-down counters of said two incremental type analog-digital converters,means for computing from said series of digital signals a valuecorresponding to a time difference between said analog signals by usingFourier transformation, controlled latches for forwarding said series ofdigital signals to said computing means, and control means forcontrolling said latches in synchronism with at least one of said analogsignals.
 2. The apparatus according to claim 1 wherein said controlmeans comprises a comparator having an input connected to one of saidanalog signals, a phase detector, a low pass filter and a voltagecontrolled oscillator connected in series, an input of said phasedetector being connected to an output of said comparator, furthercomprise a binary divider connected to an output of said voltagecontrolled oscillator, said binary divider having an output connected toanother input of said phase detector.
 3. The apparatus according toclaim 1 wherein each of said incremental type analog-digital convertersis designed so that

    LSB/T≧2·√2·π·U·f,

where LSB is the least significant bit of the up-down counter, T is thetime period of the clock signal of said clock signal source, U is ther.m.s. value of the greater one of said analog signals, and f is thefrequency of said analog signals.
 4. The apparatus according to claim 1further comprising vibrating means for oscillating said at least oneflow conduit, said vibrating means being driven by a signal which is inphase with the sum of said analog signals.